I. Field of the Invention
The present invention relates to data communication. More particularly, the present invention relates to a novel and improved method and apparatus for performing fast closed-loop rate adaptation in a high rate packet data transmission.
II. Description of the Related Art
Mobile computing and data access is steadily becoming available to an increasing number of users. The development and introduction of new data services and technologies that will provide continuous data connectivity and full access to information is presently occurring. Users can now use a variety of electronic devices to retrieve voice or data information stored on other electronic devices or data networks. Some of these electronic devices can connect to data resources through wires and some can connect to data resources through wireless solutions. As used herein, an access terminal is a device providing data connectivity to a user. An access terminal may be coupled to a computing device, such as a desktop computer, a laptop computer, or a personal data assistant (PDA), or it may be physically incorporated into any such devices. An access point is equipment that provides data connectivity between a packet switched data network and access terminals.
An example of an access terminal that can be used to provide wireless connectivity is a mobile telephone that is part of a communication system capable of supporting a variety of applications. One such communication system is a code division multiple access (CDMA) system which conforms to the “TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System”, hereinafter referred to as the IS-95 standard. The CDMA system allows for voice and data communications between users over a terrestrial link. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, both assigned to the assignee of the present invention and incorporated by reference herein. It should be understood that the present invention is equally applicable to other types of communication systems. Systems utilizing other well-known transmission modulation schemes such as TDMA and FDMA as well as other spread spectrum systems may employ the present invention.
Given the growing demand for wireless data applications, the need for very efficient wireless data communication systems has become increasingly significant. The IS-95 standard is capable of transmitting traffic data and voice data over the forward and reverse links. A method for transmitting traffic data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled “METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION,” assigned to the assignee of the present invention and incorporated by reference herein. In accordance with the IS-95 standard, the traffic data or voice data is partitioned into code channel frames which are 20 msec wide with data rates as high as 14.4 Kbps.
A significant difference between voice services and data services is the fact that the former imposes stringent and fixed delay requirements. Typically, the overall one-way delay of speech frames must be less than 100 msec. In contrast, the data delay can become a variable parameter used to optimize the efficiency of the data communication system. Specifically, more efficient error correcting coding techniques which require significantly larger delays than those that can be tolerated by voice services can be utilized. An exemplary efficient coding scheme for data is disclosed in U.S. patent application Ser. No. 08/743,688, entitled “SOFT DECISION OUTPUT DECODER FOR DECODING CONVOLUTIONALLY ENCODED CODEWORDS,” filed Nov. 6, 1996, now U.S. Pat. No. 5,933,462, issued Aug. 3, 1999, assigned to the assignee of the present invention and incorporated by reference herein.
Another significant difference between voice services and data services is that the former requires a fixed and common grade of service (GOS) for all users. Typically, for digital systems providing voice services, this translates into a fixed and equal transmission rate for all users and a maximum tolerable value for the error rates of the speech frames. In contrast, for data services, the GOS can be different from user to user and can be a parameter optimized to increase the overall efficiency of the data communication system. The GOS of a data communication system is typically defined as the total delay incurred in the transfer of a predetermined amount of data, hereinafter referred to as a data packet.
Yet another significant difference between the voice services and data services is that the former requires a reliable communication link which, in the exemplary CDMA communication system, is provided by soft handoff. Soft handoff results in redundant transmissions from two or more base stations to improve reliability. However, this additional reliability is not required for data transmission because the data packets received in error can be retransmitted. For data services, the transmit power used to support soft handoff can be more efficiently used for transmitting additional data.
The transmission delay required to transfer a data packet and the average throughput rate of a communication system are parameters that measure the quality and effectiveness of the data communication system. Transmission delay does not have the same impact in data communication as it does for voice communication, but it is an important metric for measuring the quality of the data communication system. The average throughput rate is a measure of the efficiency of the data transmission capability of the communication system.
It is well known that in cellular systems, the signal-to-interference-and-noise ratio (SINR) of any given user is a function of the location of the user within the coverage area. In order to maintain a given level of service, time division multiple access (TDMA) and frequency division multiple access (FDMA) systems resort to frequency reuse techniques, i.e. not all frequency channels and/or time slots are used in each base station. In a CDMA system, the same frequency allocation is reused in every cell of the system, thereby improving the overall efficiency. The SINR measured at any given user's mobile station determines the information rate that can be supported for this particular link from the base station to the user's mobile station. Given the specific modulation and error correction method used for the transmission, a given level of performance is achieved at a corresponding level of SINR. For an idealized cellular system with hexagonal cell layouts and utilizing a common frequency in every cell, the distribution of SINR achieved within the idealized cells can be calculated.
In a system that is capable of transmitting data at high rates, which will be referred to hereafter as a High Data Rate (HDR) system, an open-loop rate adaptation algorithm is used to adjust the data rate of the forward link. An exemplary HDR system is described in U.S. patent application Ser. No. 08/963,386, entitled “METHOD AND APPARATUS FOR HIGH RATE PACKET DATA TRANSMISSION,” now U.S. Pat. No. 6,574,211, issued Jun. 3, 2003, assigned to the assignee of the present invention and incorporated herein by reference. The open-loop rate adaptation algorithm adjusts the data rate in accordance with the varying channel conditions typically found in a wireless environment. In generally, an access terminal measures the received SINR during periods of pilot signal transmissions on the forward link. The access terminal uses the measured SINR information to predict the future average SINR over the next data packet duration. An exemplary prediction method is discussed in U.S. patent application Ser. No. 09/394,980, entitled, “SYSTEM AND METHOD FOR ACCURATELY PREDICTING SIGNAL TO INTERFERENCE AND NOISE RATIO TO IMPROVE COMMUNICATIONS SYSTEM PERFORMANCE,” now U.S. Pat. No. 6,426,971, issued Jul. 30, 2002, assigned to the assignee of the present invention and incorporated herein by reference. The predicted SINR determines the maximum data rate that can be supported on the forward link with a given probability of success. Hence, the open-loop rate adaptation algorithm is the mechanism by which the access terminal requests an access point to transmit the next packet at the data rate determined by the predicted SINR. The open-loop rate adaptation method has proven to be very effective in providing a high throughput packet data system even in adverse wireless channel conditions, such as a mobile environment.
However, the use of an open-loop rate adaptation method is impaired by the implicit feedback delay associated with the transmission of the rate request feedback to the access point. This implicit delay problem is exacerbated when channel conditions change rapidly, thus requiring the access terminal to update its requested data rate several times per second. In a typical HDR system, the access terminal would make approximately 600 updates per second.
Other reasons exist for not implementing a pure open-loop rate adaptation method. For example, the open-loop rate adaptation method is highly dependent upon the accuracy of the SINR estimate. Hence, imperfect SINR measurements would prevent the access terminal from making a precise characterization of the underlying channel statistics. One factor that would lead to imprecise channel statistics is the feedback delay discussed above. Due to the feedback delay, the access terminal must predict a supportable data rate in the near future using past and present noisy SINR estimates. Another factor that would lead to imprecise channels statistics is the unpredictable, bursty nature of received data packets. In a packet data cellular system, such bursts cause sudden changes in the interference levels seen at the access terminal. The unpredictability of the interference levels cannot be efficiently accounted for by a pure open-loop rate adaptation scheme.
Another reason for not implementing a pure open-loop rate adaptation method is an inability to minimize the effects of errors. For example, when the prediction error for an estimated SINR is large, as in the case of some mobile environments, the access terminal will transmit a conservative data rate request in order to ensure a low packet error probability. A low packet error probability will provide low overall delays in the transmission. However, it is probable that the access terminal could have successfully received a higher data rate packet. There is no mechanism in the open-loop rate adaptation method to update a data rate request based on estimated channel statistics with a data rate based on the actual channel statistics during the transmission of a data packet. Hence, the open-loop rate adaptation method would not provide a maximized throughput rate when the prediction error for an estimated SINR is large.
Another example in which the open-loop rate adaptation method fails to minimize the effects of an error is the instance when the access terminal has incorrectly decoded a received packet. The Radio Link Protocol (RLP) requires a retransmission request when the access terminal incorrectly decodes a packet, but the retransmission request is generated only after detecting a gap in the received sequence number space. Therefore, the RLP protocol requires the processing of a subsequent received packet after the incorrectly decoded packet. This procedure increases the overall transmission delay. Some mechanism is needed to implement a quick retransmission of some or all of the code symbols contained in the data packet, wherein the mechanism would enable the access terminal to correctly decode the packet without incurring excessive delays.
Hence, there exists a present need to modify the open-loop rate adaptation method in order to minimize transmission delays and to maximize the throughput rate as discussed above.